Vinylaromatic compounds such as styrene and diisopropenylbenzene undergo undesired spontaneous polymerization (ie. polymerization of monomers due to random generation of free radicals in the monomers) during storage, shipping or processing. The problem of undesired polymerization of monomer is particularly acute during purification operations. Spontaneous polymerization is disadvantageous because it causes foulling of distillation column reboilers and other equipment used for processing vinylaromatic monomers. In addition, spontaneous polymerization is an economic burden that results in loss of monomer and a decrease in overall process efficiency. Thus, there is a need to inhibit the spontaneous polymerization of vinylaromatic monomers, more specifically, dialkenylbenzenes.
To prevent spontaneous polymerization of vinylaromatic monomers, it is a common practice to add to monomers certain polymerization inhibiting compounds. A wide variety of such compounds, known as polymerization inhibitors, have been used for this purpose. Sulfur has been widely used in the past to inhibit polymerization of vinylaromatic compounds. However, sulfur usage is undesirable because relatively large quantities of sulfur are required for effective polymerization inhibition. This presents a waste removal problem when the monomer is separated from the sulfur monomer mixture by distillation. The distillation bottoms product which contains high molecular weight polymer and sulfur cannot be conveniently burned due to the potential air pollution hazard caused by sulfur oxides. Thus, this material must be disposed of by burial in a waste dump.
Many chemical compounds have been developed as substitutes for sulfur in polymerization inhibiting applications. These compounds have been used as polymerization inhibitors for vinylaromatic monomers with varying degrees of success. U.S. Pat. No. 3,390,198 discloses the use of several mono- and di-alkylcatechols as polymerization inhibitors for hot styrene. U.S. Pat. Nos. 4,061,545 and 4,177,110 disclose the use of a combination of tertiary butylcatechols and phenothiazine as a polymerization inhibitor system for vinylaromatic compounds.
U.S. Pat. No. 4,409,480 is concerned with the use of compounds such as N-dialkylhydroxylamines and tertiary alkylcatechols and their synergistic effect as a stabilization system for vinylaromatic compounds. U.S. Pat. No. 4,389,285 discloses an improved method for preparing and processing ethylenically unsaturated aromatic monomers, in that 3,5-dinitrosalicylic acid as used as a process inhibitor.
The styrene industry currently employs 4,6-dinitro-ortho-cresol as a process inhibitor. However, this compound is a highly toxic material. The toxicity of this chemical raises exposure concerns with regard to monomer production and the preparation of the chemical itself. Therefore, there exists a need in the industry for a process inhibitor to replace those presently accepted process inhibitors that are toxic or have other shortcomings.
Vinylaromatic monomers such as styrene, alpha-alkylstyrene, vinyltoluene, divinylbenzene, meta- and para-diisopropenylbenzene are important for their ability to form useful polymeric materials. These compounds are typically prepared by catalytic dehydrogenation of alkylaromatic compounds having corresponding carbon chains. The crude product of the dehydrogenation reaction, however, is a mixture of materials comprising in addition to the desired vinyl or divinylaromatic monomer various alkylaromatic compounds. These other substances must be separated from the monomer to obtain a commercially useful product.
It has now been discovered that dithiocarbamates, more specifically, zinc or nickel dialkyldithiocarbamates provide outstanding polymerization inhibiting activity for vinylaromatic monomers, more specifically, dialkenylbenzenes. Thus, because of the properties of zinc or nickel dialkyldithiocarbamate, it is now possible to provide superior polymerization inhibiting protection over those materials presently accepted in the industry.
The present invention provides stable compositions of vinylaromatic monomers and a method for effectively and economically inhibiting spontaneous polymerization of styrene, vinylaromatic monomers and, more specifically, meta- and para-diisopropenylbenzenes.